1. Technical Field
The present invention relates to an optical head for optical information processing, optical communication and the like. More specifically, the present invention relates to an optical head for an optical recorder/reproducer.
2. Description of the Related Art
Recently, a Digital Versatile Disc (DVD) has attracted attention as a large-capacity optical recording medium, which records digital information with a record density six times denser than that of a compact disc (CD). However, as the amount of information to be stored increases, there has arisen a demand for an even higher density optical recording medium. To achieve a density which is higher than that of DVD (wave length of 660 nm and numerical aperture (NA) of 0.6), it is necessary to shorten a wave length of a light from a light source or to enlarge an NA of an objective lens. When a blue laser of 405 nm wavelength is used as a light source and an objective lens of 0.85 NA is used, for example, it is possible to achieve a record density of digital information recorded on an optical recording medium that is five times denser than that of DVD.
JP-A-2000-131603 discloses an optical head which may be used with a blue laser light source to reproduce and record digital information.
Below, the conventional optical head mentioned above is described with reference to the attached drawings.
FIG. 12 is a schematic diagram of the conventional optical head wherein: 91 refers to a polarization beam splitter; 92 refers to a quarter wave plate; 93 refers to a spherical aberration correcting optical device; 94 refers to an objective lens; 95 refers to an optical recording medium; 96 refers to a focusing lens; 97 refers to a multilens; and 98 refers to a photodetector.
Incident light may be, for example, blue light having a wavelength less than or equal to 450 nm which is output from a gallium nitride series laser and collimated. The polarization beam splitter 91 is an optical device which shows different transmittance and reflectance according to a polarization direction of incident light. The polarization beam splitter is used to split the light. The quarter wave plate 92 is an optical device made of a birefringent material. The quarter wave plate 92 is used to convert linearly-polarized light to circularly-polarized light. The spherical aberration correcting optical device 93 is an optical device for correcting a spherical aberration developed by a deviation of a thickness of a substrate of the optical recording medium 95 from a predetermined optimal substrate thickness. The spherical aberration correcting optical device 93 includes a concave lens, a convex lens, and a uniaxial actuator (not shown), and can correct the spherical aberration by changing the distance between the concave lens and the convex lens. The objective lens 94 condenses light onto a recording layer of the optical recording medium and is configured with two groups two lenses composition. The focusing lens 96 condenses the light which is reflected on the recording layer of the optical recording medium onto the photodetector 98. The multilens 97 has a cylindrical plane as an incidence plane and a plane which is rotationally symmetric with respect to the optical axis of the multilens as an emittance plane and provides the incident light with astigmatism, which enables the detection of a focus error signal by using the so-called astigmatism method. The photodetector 98 receives the light which is reflected by the recording layer of the optical recording medium 95 and converts the light to electrical signals.
Next, operation of the optical head thus configured will be described. Parallel blue light which is emitted from a gallium nitride laser transmits through the polarization beam splitter 91 and enters the quarter wave plate 92. The quarter wave plate 92 converts the linearly-polarized light to circularly-polarized light. Then the light which is transmitted through the quarter wave plate 92 enters the spherical aberration correcting optical device 93. The optical device 93 converts the incident parallel light to divergent light or convergent light by changing the distance between the concave lens and the convex lens, both of which are constituting the spherical aberration correcting optical device 93, so as to correct for spherical aberration that occurs when a thickness of the substrate of the optical recording medium 95 deviates from its optimal substrate thickness. This converted light enters the objective lens 94 and is imparted with a spherical aberration on some level according to a degree of divergence or convergence of the incident light. And then, the converted light is focused on the optical recording medium 95. Thus, the light which is prospectively imparted with a wavefront aberration enters objective lens 94 to be condensed so that a wavefront aberration due to a deviation of the substrate thickness from the optimal substrate thickness may be corrected. Therefore, a spot of the light which has no aberration is formed on the optical recording medium 95, that is, the light is focused to a diffraction limit on the medium 95. The circularly-polarized light subsequently reflected on the optical recording medium 95 enters the quarter wave plate 92 through the spherical aberration correcting optical device 93. The quarter wave plate 92 converts the reflected light to linearly-polarized light of which polarization direction is perpendicular to the polarization direction of the laser light propagating toward the optical recording medium 95. The linearly-polarized light, which is converted by the quarter wave plate 92, reflects on the polarization beam splitter 91 and enters the focusing lens 96. This light is focused by the focusing lens 96, imparted astigmatism by the multilens 97, and condensed on the photodetector 98. The photodetector 98 outputs a focus error signal which indicates a condition of focusing of the light on the optical recording medium 95 and a tracking error signal which indicates the irradiation position of the light on the optical recording medium 95. Then the focus error signal and the tracking error signal are detected by means of well-known methods such as the astigmatism method and the three beams method. A focus controller (not shown) controls the position of the objective lens 94 along its optical axis according to the focus error signal so that the light condensed may constantly be focused on the optical recording medium 95. A tracking controller (not shown) also controls the position of the objective lens 94 according to the tracking error signal so that the light may be condensed on the desired track of the optical recording medium 95. In addition, the photodetector outputs information recorded on the optical recording medium 95. Thus configured, the spherical aberration due to the deviation of the substrate thickness of the optical recording medium 95 from the optimal substrate thickness can be corrected using the spherical aberration correcting optical device 93.
The optical head thus configured can be used to reproduce and record digital information even when light of less than or equal to 450 nm wavelength is used as a light source.